The present invention relates to a light-emitting device (LED), and more particularly to an interface texturing for a LED and a process of making the same, which can improve lightness of the LED.
In recent years, some kinds of light-emitting device (LED) are developed and applied in flat-panel displayer (FPD). Among the LEDs, semiconductor light-emitting diodes are rapidly developed and generally used in indoor and outdoor displaying.
FIG. 1 shows a cross-sectional view of semiconductor light-emitting diodes. An epitaxial layer 20 having a p-n junction active layer 24 and window layers 22 and 26 is stacked on a semiconductor substrate 10. The epitaxial layer 20 is usually made of gallium phosphide (GaP) or a material selected from the group of gallium phosphide, such as gallium arsenide (GaAs). Electrode 30 are formed on the top and bottom of the stack layer. By injecting electric current, the p-n junction active layer is xe2x80x9cactivizedxe2x80x9d and emitted, and thereby a light beam L is ejected out.
According to light refracting law, while a light beam in medium (I) ejects to medium (II), it must satisfy phase-matching condition to allow power transmission. That is, it must satisfy sin(xcex81)*n1=sin(xcex82)*n2, wherein xcex81 and xcex82 are the incident angle to the interface, and n1 and n2 are the material index of refraction. Otherwise, reflection will be occurred and the light beam can not be transmitted into the medium (II). When the refraction index of medium (I) is greater than medium (II), the incident angle xcex8 must smaller than critical angle xcex8C=arcsin(n2/n1), or else the total internal reflection will be occurred and the light beam does not propagate into medium (II).
For semiconductor LED, the semiconductor material has refraction index (nxcx9c2.2-2.8) much greater than ambient, such as air (nxcx9c1) or transparent epoxy (nxcx9c1.5). When the light beam L from the semiconductor LED propagates to ambient and has an incident angle xcex8 greater than critical angle xcex8C, total reflection is happened thereby limiting the external quantum efficiency of the semiconductor LED.
As shown in FIG. 1, the active layer 24 is emitted and generates light beam L. For example, GaP (n1xcx9c3.3) and epoxy (n2xcx9c1.5) are used. Light beam L can be transmitted through the interface between GaP and epoxy layer if the incident angle xcex8 smaller than critical angle xcex8c, else light beam L will be total reflected to light beam Lxe2x80x2 and be again total reflected to light beam Lxe2x80x3. Therefore, the lights beam L will be continuously total reflected in the epitaxial layer 20, and finally be absorbed under the reflection path or escaped from the sidewall.
For critical angle xcex8c=27xc2x0, isotropic point source of light within the GaP, the fraction of light emitted into the epoxy is only (1xe2x88x92cos xcex8c)/2=5.5% of the available emitted light. For a cubic-shaped device having a completely reflective bottom surface, no top contact, and no internal absorption, there are six such interfaces and the fraction of total emitted light escaping the LED is 6xc3x975.5%=33%. There is still a wide range for improving the extraction efficiency.
Hence, several methods for improving the light extraction from an LED have been proposed. One method is to change the macroscopic geometry of the LED to allow all or most of the light generated within the device to enter an escape cone at the interface with the ambient. Carr in Infrared Physics 6. 1 (1996) observed that truncated cones, truncated pyramids, etc. can improve extraction efficiency. Dierschke, et al. in Applied Physics Letters 19.98 (1971) also noted large improvements in extraction efficiency for a hemispherical device. However, macroscopic shaping methods are costly and have associated manufacturability issues such as inefficient material utilization and complicated fabrication processes and techniques.
In additional, Arpad, et al. in U.S. Pat. No, 3,739,217 described that another method is random texturing or roughening of the surfaces of the semiconductor LED, as shown in FIG. 2. This randomization increases the overall probability that light L will enter an escape cone after many multiple passes through the device structure. But, each random texturing of the surfaces of the semiconductor LED is different, and much light is absorbed before escaping. These result in the extraction efficiency of each semiconductor LED is hardly controlled.
The present invention provides an interface texturing for a light-emitting device (LED). An ordered interface texturing is formed in the LED by using holographic lithographic techniques. The incident angle of the light total reflected in the textured interface can be changed in next time, and the probability of transmission in the interface can be improved. Therefore the total extraction efficiency can be increased.
The present invention provides a method of fabricating an interface texturing for a light-emitting device. The method comprises the step of providing a substrate; forming a photoresist layer over the substrate; performing at least one step of exposure, wherein interference lines formed by two overlaid coherent lights are projected to the photoresist layer; performing a step of develop to form a textured pattern on the surface of the photoresist layer; and performing an etching step to transfer the photoresist pattern to the substrate.
The present invention also provides a light-emitting device comprising: a luminescent layer having a textured surface, wherein the profile of the textured surface is caused by at least one projection of light interference lines. The luminescent layer can be such as an epitaxial layer of a semiconductor LED.